Loss of ovarian function following menopause results in a substantial increase in bone turnover and a critical imbalance between bone formation and resorption. This imbalance leads to progressive loss of trabecular bone mass and significantly increases the risk of atraumatic fracture during the aging process. Estrogen (E) treatment prevents bone loss following the menopause. The cellular targets of E action in bone include differentiated osteoblasts and osteoclasts as well as their bone marrow-derived respective mesenchymal and hematopoietic cell precursors. In this application the Principal Investigator proposes to examine the stromal cell-independent actions of estrogen to repress both lineage progression and osteoclast differentiation. In Specific Aim 1, the investigators will focus on E action in myeloid/lymphoid lineage progression, an effect that limits pools of cells capable of either becoming osteoclasts or of modulating osteoclastogenesis. They will characterize the anti-proliferative, pro-apoptotic effects of E on IL-3 and GM-CSF dependent proliferation of lymphoid and myeloid cells. They will utilize transcriptional responses to determine the nature of the signaling pathways that are E repression targets and identify the transcription factors involved. They will also assess the mechanisms by which ER mediates repression by defining the ER repression domains, determining the site of intervention within the activation pathway and assessing the nature of the molecular interactions that occur. Finally they will evaluate the actions of raloxifene and tamoxifen in this system. In Specific Aim 2, studies will focus on E action during osteoclast differentiation, examining E anti-differentiative, pro-apoptotic effects in primary murine cells and in M-CSF/RANKL dependent OC cell models. They will determine which components of the RANKL signaling pathway represent targets of E action using selected transcriptional responses and then identify the transcription factors involved. They will characterize the mechanism of repression by determining whether E intervention is proximal to or distal to transcription and then define the molecular interaction. As in Specific Aim 1, the activity of SERMs in osteoclast differentiation will be evaluated.